[Feature request] no_std support

[jschneider-bensch]

Is your feature request related to a problem? Please describe.

It would be nice to be able to run Bertie on embedded devices. Currently that's blocked by the implementation of Bytes being backed by a std::Vec.

Describe the solution you'd like

We could continue to use Vec and ask the application to provide an allocator for it.

Describe alternatives you've considered

Perhaps we can also use work coming out of cryspen/libcrux#453 in this context.

[franziskuskiefer]

We should also switch to using the pure Rust version from cryspen/libcrux#648. I think we are only missing P256 support to be able to switch.